Deep water waves in lakes

The properties of waves on a lake surface and the influence of water depth on these properties are discussed. The method, derived by the Beach Erosion Board, of predicting wave characteristics if the wind speed and fetch are known is outlined and the validity of the method in Loch Leven, Scotland, is confirmed. This prediction technique is then combined with the equations of classical hydrodynamics to form a method of assessing the nature and extent of wave action in lakes without direct measurement. The value of the ratio of the wave-mixed volume to the total lake volume as a meausre of hydraulic conditions in lakes is suggested.     

On the dynamical response of Lake Chapala, Mexico to lake breeze forcing

Fluctuations in the atmospheric characteristics, as well as variations in the water level of Lake Chapala are discussed. Field measurements of the atmospheric characteristics and lake level during December 1996 through January 1997 are described; using spectrum analysis of synchronous time series. The findings suggest that the variability is due to the diurnal cycle of atmospheric elements. Lake breeze circulation plays an important role in the area of Lake Chapala; since it was registered in 83% of the data. Periodic fluctuations in atmospheric pressure and wind generate significant seiche amplitudes in the lake, with the periods of about 6 h. With the help of a simple model, the seiche parameters are estimated. The amplitude of one-nodal seiches on one of the edges of the lake; is on average equal to 18 mm. This wave should generate currents of approximately 0.012 m s^–1 at the lake`s centre in the area of the nodal line. The experimental results on the thermal regime and circulation of Lake Chapala are discusssed as well. Surface temperature variations were registered at the eastern part of the lake. In all cross-sections, typical spatial variations of 3 °C were registered, over a distance of 100–300 m. A bouy station registered movements of an internal thermal front in the body of the water. The leading edge of the front was accompanied by intense internal waves, in the form of internal KdeV solitones. The front near the buoy station was produced by the movement of a warm body of water travelling from the shallow eastern part of the lake and trigered by morning breeze.     

Modelling spatial distributions of Ceratium hirundnella and Microcystis. in a small productive British lake

The short-term relationships between the spatial distributions of phytoplankton and the environmental conditions of Esthwaite Water, a small eutrophic lake in the English Lake District, UK, were examined using a hydrodynamic model. Spatial distributions of phytoplankton were simulated on two occasions the first, when the population was dominated by dinoflagellates; and the second, when the population was dominated by cyanobacteria.Vertical motility of the dinoflagellate Ceratium hirundinellaand buoyancy of the cyanobacteria Microcystis ssprm.were estimated as functions of irradiance. Water velocity fields were estimated through solving the 3-D Navier–Stokes equations on a finite-volume, unstructured non-orthogonal grid. Simulated circulation patterns of water and phytoplankton were similar to those obtained through field observations. Near-surface drift currents were initiated by wind stress, which then generated return currents along the seasonal thermocline. Aggregations of motile Ceratiumthat existed near the thermocline were pushed upwind by the deep return currents and accumulated at upwelling areas. In contrast, near-surface aggregations of Microcystiswere pushed downwind by the surface currents and accumulated at downwelling areas. Horizontal and vertical phytoplankton distributions resulted from the interaction between the vertical motility of the phytoplankton (dependent upon the light environment) and the velocity vectors at the depths at which the phytoplankton accumulated (dependent upon wind stress and morphometry). Modelling showed that phytoplankton motility and buoyancy greatly affect phytoplankton spatial distributions.     

Field and laboratory methods to monitor lake aerosols for cyanobacteria and microcystins

This study tested field and laboratory methods for the collection of cyanobacteria and microcystins emitted from lake water. These methods feature a highly portable, on-lake system for collecting aerosols directly from the lake, as well as a laboratory system for measurement of aerosols from freshly collected water samples under controlled conditions. Membrane air filters (0.45 μm) collected small particles such as picoplankton (0.2–2.0 μm) from aerosolized lake water. Picocyanobacteria were distinguished from other photosynthetic cells with epifluorescence microscopy using excitation filters for chlorophyll a (435 nm) and for phycobilin pigments (572 nm), characteristic of cyanobacteria. Aerosolization of picocyanobacteria ranged from 8872 to 167,297 cells m ^?3 in the field and 23,764 to 365,011 cells m ^?3 in the laboratory. Microcystin levels from field air filters ranged (below detectable limits) <13–384 pg MC m ^?3 of air. The described methods could be used for monitoring aerosolized cyanobacteria for public health purposes.     

Aspects of the spatial distribution of the algae of the nant-y-moch reservoir,Ceredigion, Wales

The spatial distribution of algae in the Nant-y-Moch reservoir has been studied in relation to a number of factors. It was not possible to relate the variation in algal numbers to the wind parameters studied and it is postulated that water currents, set up by the abstraction of water, may be influencing the distribution. The variation of species composition in the different parts of the lake was summarised by means of diversity and heterogeneity indices. It was found difficult to relate the differences in diversity indices to the position of the transects sampled; which again was probably the result of the complex water patterns likely to exist in the lake. Average mean heterogeneity indices indicated that four transects were relatively different from each other and from the other transects studied; reasons for the differences are suggested.     

Wind distribution of the egg masses of Chironomus anthracinus (Zetterstedt) (Diptera: Chironomidae) in a shallow, wind-exposed lake (Loch Leven, Kinross)

A mass emergence of Chironomus anthracinus (Zetterstedt) took place at Loch Leven, Kinross, in late May and early June 1971, leading to a large concentration of egg masses. The distribution of egg masses within the lake was determined on 6 June 1971 and the accumulation of egg masses along wind-exposed shores, under the influence of a 17–23 km h^?1 wind, was noted. Oviposition site selection by the weak flying adults appears to have little importance upon the final distribution of larvae within the lake. Temporary planktonic activity of first instar larvae under the influence of wind-induced water currents is briefly discussed as a dispersal mechanism in the light of observed egg mass accumulation in the lake.     

Circulation patterns; their investigation and prediction

The effect of wind on the water currents in Rutland Water is being studied with a view to predicting algal distribution patterns so that, by exercising different abstraction options, water treatment problems can be avoided. Investigations using a scale, physical model of the reservoir in a wind tunnel suggest that under most wind directions the circulations tend to separate the water body into three distinct, topographic regions. Water currents in the reservoir itself have been plotted by tracking the paths of drogues using theodolites. This has indicated that in the central basin the circulation may be of the conveyor belt type, rather than the solely horizontal type of the model. Surface chlorophyll readings are shown to indicate wind-driven water currents. Algal distribution has been predicted from wind data; however, the analysis requires modification in order to apply to all wind conditions. The effect of wind on the vertical distribution of algae and its implication for abstraction are discussed.     

Redistribution of sediments in three Swedish lakes

Sedimentation and redistribution of fine sediments in three Swedish lakes of different character have been investigated using settling sediment traps. The bottom shear stress from wind generated waves are calculated and the extension of erodable bottom area is related to wind conditions. Wave induced erosion and deposition during and after cessation of storms in different parts of a lake are discussed theoretically. It is shown that a single one year storm may redistribute more bottom material than the accumulated resuspension caused by frequent but smaller wind events. The settling sediment trap deposition and the concentration of suspended solids are related to the extension of erodable bottom area of particular storms. It is found that in lakes where there are relatively large areas of erosion bottoms, resuspended material from the part of the lake most susceptible to strong winds of large fetch constitutes a major part of the settled material on deep bottoms.     

The trophic history of Enäjärvi,SW Finland,with special reference to its restoration problems

The eutrophicated Enäjärvi was studied by paleolimnological analyses and sediment mapping. The sedimentary record indicates that the lake nutrient balance had deteriorated due to lowering of the lake water level in the year 1928. From that event onwards Chironomus plumosus and Cyclotella astraea characterize the chironomid and diatom communities. The concentrations of sedimentary total and mobile phosphorus show that since then the internal load of phosphorus has controlled the nutrient cycle of the lake. The areal distribution of mobile phosphorus can be explained by dominant wind directions and wind resuspension of the sediment is the key factor in the nutrient cycle. Restoration of Enäjärvi must be based on actions which stabilize the surface sediment and improve its natural phosphorus-binding capacity. They include the regulation of lake water to as high a level as possible and the removal of the majority of roach.     

Modeling impacts of Yangtze River water transfer on water ages in Lake Taihu, China

To improve water quality and alleviate eutrophication in Lake Taihu, the third largest freshwater lake in China, a Yangtze River water transfer project was initiated in 2002 to bring water from the Yangtze River to Lake Taihu to dilute and divert pollutants out of the lake. We used a three-dimensional numerical model, Environmental Fluid Dynamics Code, to study the impacts of water transfer on the transport of dissolved substances in the lake by using the concept of water age. In particular, the influences of inflow tributaries and wind forcing on water age were investigated. Model results showed that the effect of water transfer on transport processes in the lake is strongly influenced by hydrodynamic conditions induced by wind and inflow/outflow tributaries. During the simulation year (2005), the water ages in Lake Taihu were highly variable both spatially and temporally, with a mean of approximately 130 days in summer and 230 days in the other seasons. Southeasterly winds—dominant in the summer—could improve the quality of water by reducing the water age in the eastern areas of the lake, which are used as a drinking water source, and in Meiliang Bay, the most polluted bay. In terms of dilution, the most efficient flow rate for transferred water was predicted to be approximately 100 m³/s. The spatial distribution of water ages showed that water transfer may preferentially enhance exchanges in some areas of the lake unless nutrient concentrations in the transferred water are reduced to a reasonable level. This study provides useful information for a better understanding of the complex hydrodynamic and mass transport processes in the lake, which is important for developing and implementing effective ecological restoration strategies in the region.     

Water currents and spreading of river load in Lake Pyhäselkä, Saimaa, Finland

During the summers of 1989–1990, the pattern of water flow and the water quality in Lake Pyhäselkä, Finland, were studied using field observations and a 3-dimensional model. The lake receives its major point-source loading, including pulp mill effluents, via the Pielisjoki River in the northeastern part of the lake, where the water is slightly eutrophic. The flow pattern was measured with three types of recording current meters, two of which were conventional mechanical meters and the third a new acoustic current profiler. The profiler was used in direct reading mode on a research vessel and also in self-contained mode on a float installed in the deep basin of the lake. The water-quality part of the model simulates oxygen, BOD, total phosphorus, AOX, and phytoplankton biomass. The three-dimensional model applied here simulates accurately the flow pattern of the water in Lake Pyhäselkä. The results of this study made it possible to detect different flow patterns in the lake. The main load is clearly affected by the wind-induced pattern of flow.     

Possible effects of global climate change on the ecosystem of Lake Tanganyika

Any change in the air temperature, wind speed, precipitation, and incoming solar radiation induced by increasing greenhouse gasses and climate change will directly influence lakes and other water bodies. The influence can cause changes in the physical (water temperature, stratification, transparency), chemical (nutrient loading, oxygen) and biological (structure and functioning of the ecosystem) components of the Lake. In this work an influence of the likely effects of the climate change on the above three components of Lake Tanganyika are studied by means of a simple ecological model. Simulations for the years 2002–2009 have been performed using the wind and solar radiation data from the National Centres for Environmental Protection (NCEP) reanalysis. Various possible climatic scenarios are studied by changing the surface layer depth, its temperature and the wind stress. Any change in any of the above physical forcing parameters modifies the timing and intensity of the dry season peaks of the biogeochemical parameters. It is seen that the gross production increases as temperature of the surface layer increases and its depth decreases. High temperature and low wind stress, reduces the biomass. The effects of a slight increase in lake water temperature on the Lake Tanganyika ecosystem might be mitigated by increased windiness, if the latter was sufficient to induce greater mixing.     

Basin-scale control on the phytoplankton biomass in Lake Victoria, Africa

The relative bio-optical variability within Lake Victoria was analyzed through the spatio-temporal decomposition of a 1997–2004 dataset of remotely-sensed reflectance ratios in the visible spectral range. Results show a regular seasonal pattern with a phase shift (around 2 months) between the south and north parts of the lake. Interannual trends suggested a teleconnection between the lake dynamics and El-Niño phenomena. Both seasonal and interannual patterns were associated to conditions of light limitation for phytoplankton growth and basin-scale hydrodynamics on phytoplankton access to light. Phytoplankton blooms developed during the periods of lake surface warming and water column stability. The temporal shift apparent in the bio-optical seasonal cycles was related to the differential cooling of the lake surface by southeastern monsoon winds. North-south differences in the exposure to trade winds are supported by the orography of the Eastern Great Rift Valley. The result is that surface layer warming begins in the northern part of the lake while the formation of cool and dense water continues in the southern part. The resulting buoyancy field is sufficient to induce a lake-wide convective circulation and the tilting of the isotherms along the north-south axis. Once surface warming spreads over the whole lake, the phytoplankton bloom dynamics are subjected to the internal seiche derived from the relaxation of thermocline tilting. In 1997–98, El-Niño phenomenon weakened the monsoon wind flow which led to an increase in water column stability and a higher phytoplankton optical signal throughout the lake. This suggests that phytoplankton response to expected climate scenarios will be opposite to that proposed for nutrient-limited great lakes. The present analysis of remotely-sensed bio-optical properties in combination with environmental data provides a novel basin-scale framework for research and management strategies in Lake Victoria.     

Physical and Chemical Limnology of a Subsaline Athalassic Lake in West Greenland

Physical and chemical profiles of a shallow (c. 12-m-deep) subsaline (total dissolved solids 2.3-2.8 g l^–1) closed-basin lake in the continental area of southwestern Greenland are described for the first time. Watercolumn data for every 5th consecutive day between April 20 and October 6, 2001, and continuous recordings of lake water level and meteorological conditions are used to infer controls on contemporary lake functioning, sediment formation and climate-lake interactions. Limnological observations demonstrate the importance of lake-ice formation and its role in haline convection and the development of meromixis. Observed lake cycling suggest that the lake at present is in a state of near-meromixis where stagnant bottom waters de-stratify through deep penetration of weak haline convective cells by the end of June. From this study, the primary reasons the shallow Greenlandic low salinity lakes develop meromixis are:(i) lack of an outflow (ii) meltwater dilution and chemical stratification of surface waters, (iii) insubstantial wind mixing, (iv) a weak winter thermohaline convective cell forced by cryoconcentration, and (v) biogeochemically enhanced solute concentrations near the sediment bed. Throughout the open water period the hydrological balance is dominated by evaporative losses. Lake surface water conductivities change from 2110 to 2890 S cm^–1 due to the combined effects of open water evaporation, meltwater dilution, diffusive exchanges over the seasonal pycnocline, and boundary mixing. Freeze-out of salts and resulting deep haline convection increase overall water column salinity during winter. Owing to deep convective mixing, plant nutrients are relatively high in the upper watercolumn with a dominant internal source of phosphorous. Extreme productivity pulses of phytoplankton are observed as soon as sub-ice radiation levels increase and directly after ice-out when sufficient wind mixing can support an intense monospecific diatom bloom of Diatoma spp. leading to the rapid depletion of dissolved silica.     

Interaction between Water and Wind as a Driver of Passive Dispersal in Mangroves

Although knowledge on dispersal patterns is essential for predicting long-term population dynamics, critical information on the modalities of passive dispersal and potential interactions between vectors is often missing. Here, we use mangrove propagules with a wide variety of morphologies to investigate the interaction between water and wind as a driver of passive dispersal. We imposed 16 combinations of wind and hydrodynamic conditions in a flume tank, using propagules of six important mangrove species (and genera), resulting in a set of dispersal morphologies that covers most variation present in mangrove propagules worldwide. Additionally, we discussed the broader implications of the outcome of this flume study on the potential of long distance dispersal for mangrove propagules in nature, applying a conceptual model to a natural mangrove system in Gazi Bay (Kenya). Overall, the effect of wind on dispersal depended on propagule density (g l^-1). The low-density Heritiera littoralis propagules were most affected by wind, while the high-density vertically floating propagules of Ceriops tagal and Bruguiera gymnorrhiza were least affected. Avicennia marina, and horizontally floating Rhizophora mucronata and C. tagal propagules behaved similarly. Morphological propagule traits, such as the dorsal sail of H. littoralis, explained another part of the interspecific differences. Within species, differences in dispersal velocities can be explained by differences in density and for H. littoralis also by variations in the shape of the dorsal sail. Our conceptual model illustrates that different propagule types have a different likelihood of reaching the open ocean depending on prevailing water and wind currents. Results suggest that in open water, propagule traits (density, morphology, and floating orientation) appear to determine the effect of water and wind currents on dispersal dynamics. This has important implications for inter- and intraspecific variation in dispersal patterns and the likelihood of reaching suitable habitat patches within a propagule''s viable period.     

Strategies of a Bornean tropical rainforest water use as a function of rainfall regime: isohydric or anisohydric?

Although Bornean tropical rainforests are among the moistest biomes in the world, they sporadically experience periods of water stress. The observations indicate that these ecosystems tend to have little regulation of water use, despite episodes of relatively severe drought. This water-use behaviour is often referred to as anisohydric behaviour, as opposed to isohydric plants that regulate stomatal movement to prevent hydraulic failure. Although it is generally thought that anisohydric behaviour is an adaptation to more drought-prone habitats, we show that anisohydric plants may also be more favoured than isohydric plants under very moist environments where there is little risk of hydraulic failure. To explore this subject, we examined the advantages of isohydric and anisohydric species as a function of the hydroclimatic environment using a stochastic model of soil moisture and carbon assimilation dynamics parameterized by field observations. The results showed that under very moist conditions, anisohydric species tend to have higher productivity than isohydric plants, despite the fact that the two plant types show almost the same drought-induced mortality. As precipitation decreases, the mortality of anisohydric plants drastically increases whereas that of isohydric plants remains relatively constant and low; in these conditions, isohydric plants surpass anisohydric plants in their productivity.     

Spatial decision support system for assessing lake pollution hazard: southeastern pampean shallow lakes (Argentina) as a case study

This study gives an account of the implementation of a decision support system as a logical framework for assessing lake pollution hazard. The use of this system is demonstrated with an example from two lake watersheds, each one with different land-use, soil and topographic characteristics and also management regulations for natural resource protection. Lake pollution hazard is assessed as a function of two primary topics: hydrologic and soil conditions. This model shows the state of each evaluated subwatershed with respect to its potential contribution to lake pollution hazard based mainly on easily measurable and commonly used parameters: drainage density, hydraulic gradient, water table depth, soil media, land-cover and topography. Mapped outputs from the logic model allowed the identification of several subwatersheds in each lake system as the main water resource protection areas. La Brava Lake Watershed results reasonably fit the natural conditions of the watershed, identifying those subwatersheds with a moderate to high drainage density, high hydraulic gradient and extensive agricultural activities. Regarding Los Padres Lake Watershed, moderate to high drainage density, a high to a very high hydraulic gradient, shallow water depth, silt-loam soil texture and intensive agricultural and residential lands, contributed to a high lake pollution hazard. Results highlighted differences between both freshwater systems, being Los Padres Lake Watershed a more vulnerable ecosystem, requiring decision maker’s intervention based on accurate and up-to-date information.     

Spatial isolation and fish communities in drainage lakes

Fifty-two drainage lakes, located in south-central Ontario, Canada, were examined to study the association of isolation- and environment-related factors with fish community composition. Eight quantitative measures of lake isolation were examined, each of which incorporated potential ecological "challenges" that a fish encounters when moving between lakes. A Procrustean approach was employed to assess the degree of concordance between fish assemblage structure, measures of lake isolation and environmental conditions (i.e., lake morphology and water chemistry). Our results revealed a high concordance between patterns in fish community composition and lake isolation and lake morphology at the watershed scale, suggesting that insular and habitat-related factors influence the structure of fish communities. At the scale of the individual lake, this relationship varied greatly, ranging from a strong match of community composition with both spatial and abiotic conditions to communities exhibiting weak association with these conditions. Furthermore, we showed that alternative measures of lake isolation provide additional insight into potential factors shaping patterns in fish community composition; information not provided using straight-line distances between lakes. Finally, the statistical methodology outlined in this paper provides a robust technique for assessing both the overall association between multivariate data matrices (i.e., landscape or regional scale), as well as facilitating the examination of smaller-scale relationships of individual observations (i.e., local scale).